Iran is rolling back parts of its nuclear program and getting relief from sanctions in return as an interim agreement aimed at gauging Tehran's willingness to curb its nuclear ambitions appears to be working with global powers gearing up for talks on Tuesday to forge a long-term pact.

"So far everyone, both Iran and all of the rest of us who provided some very limited, targeted sanctions relief have kept their commitments," Wendy Sherman, a senior State Department official and lead negotiator for the United States on the Iran deal, told Wolf Blitzer on Monday in an interview on CNN's "The Situation Room."

Sherman, the under secretary for political affairs, spoke from Vienna where talks on a comprehensive accord between Iran, the United States, Germany and other permanent members of the U.N. Security Council are due to begin on Tuesday.

The six-month deal was reached in November and took effect in January.

The Vienna talks "will build on that first step, because we don't want it to be the only step," Sherman said.

"We go into this negotiation very clear eyed, very sober," she said. "It's going to be very tough."

Her comments came as Ayatollah Ali Khamenei, Iran's supreme leader, said his country would not "renege" on the talks, but predicted they "will not lead anywhere."

"What I care about is what Iran does much more than what Iran says," Sherman said.

She added that any final deal will be contingent on Iran taking "concrete" verifiable steps that prevent it from ever obtaining a nuclear weapon.

The United States and its allies believe Iran is aiming to develop a nuclear weapon, while Tehran has said it's atomic efforts are peaceful.

Sherman said she had seen recent Iranian reports that Russia has offered to build a nuclear reactor inside Iran in exchange for oil shipments, but did not offer a comment on whether or not that was a good idea.

The news was devastating for Shannon and Jeremiah Collins: Their 19-year-old son, Marine Lance Cpl. Jeremiah Collins Jr., died in Afghanistan's Helmand province.

Then it got worse.

The Milwaukee, Wisconsin, couple learned the survivor benefit paid to the families of fallen troops for burial and other expenses had been suspended because of the government shutdown.

Her sadness, Shannon Collins says, was compounded by worry and questions about how to pay off the debt.

It's a question that embarrassed and outraged government officials, who scrambled to find a way to provide the survivor benefits to the families of 26 troops who have died since the shutdown began on October 1.

On Wednesday, the Pentagon struck a deal with a private charity to ensure families of fallen troops are paid the survivor benefits, which include a $100,000 payment made within days of the death, Defense Secretary Chuck Hagel said.

The government will reimburse the Maryland-based Fisher House Foundation once the shutdown is over, Hagel said in a written statement.